The present invention relates to an apparatus for aligning two objects. More particularly, the present invention relates to a detection of an alignment mark on a mask or reticle or of an alignment mark on a wafer prior to aligning a semiconductor circuit pattern on a reticle or a mask with a wafer.
A semiconductor circuits manufacturing includes steps of sequentially printing several patterns on a wafer to form an integrated semiconductor circuit. Those steps require that the wafer and the mask bearing a pattern are aligned with each other with a high precision so as to correctly align a pattern of a mask to the pattern which has been printed on the wafer in the previous step or steps. Prior to the alignment operation, alignment marks formed on the mask and the wafer are photoelectrically detected to determine a relative positional relation therebetween, and then a relative movement between the mask and the wafer is made on the basis of the positional determination.
The alignment mark of the mask or wafer is detected by a signal produced by the mark being at a peak level which is over a predetermined reference level. Heretofore, the reference level has been fixed so that it may be too high or low depending on the kind of the wafer or the mask being processed. Those improper reference levels may result in failure of the detection of the alignment marks. It is known that, when the detection fails, the reference level is manually changed.